1. Field of the Invention
The present invention is related to a data transmission system, and particularly to a data transmission system which can perform data transmission from a computer or data processing unit to another one or more computers in a simple construction and at high speed.
2. Description of the Prior Art
In automobiles, motorbicycles and the like (hereinafter simply referred to as vehicles), many functions such as spark control of the engine, fuel injection control, automatic gear change control, driving force control, brake control, suspension control, air-conditioning control and self-diagnostics are performed by an electronic control unit (hereinafter referred to as ECU) including a data processing unit such as a microcomputer. Since performance of one microcomputer is insufficient for performing such a plurality of controls, separate microcomputers are used for each control task or a group of them.
Since these plurality of microcomputers often use common vehicle data such as engine speed, vehicle speed and temperature, the load of each microcomputer is reduced if data communication is performed between these microcomputers for data sharing.
As to data communication, the technique of LAN (Local Area Network) in the usual electronic communication field is well known, but, if the system of LAN is directly applied to a carborne data transmission system, the system can be unsuitably large-scale.
On the other hand, a data transmission system for vehicles which does not use the LAN technique is also described, for instance, in the Japanese Patent Kokai 62-257239 official gazette. In this system, an input/output interface provided with a shift register is connected to each of a plurality of electronic control units, and the shift registers are serially connected to each other to form a loop transmission path, and data are sequentially transmitted in the transmission path by outputting data bit-by-bit into the shift registers.
In the above-mentioned prior art data transmission system, if it is attempted to send out data to the loop transmission path in the system, data are output bit-by-bit to the shift registers constituting the transmission path. The output data are shifted bit-by-bit in synchronism with the synchronizing signal for data transmission. In this system, data sent from a microcomputer are sent to the next microcomputer after first being stored in a register, and thus there is a transmission delay for the data length to be sent to the next microcomputer.
The data delay increases as the number of the connected microcomputers increases, and therefore the number of connectable microcomputers is limited if high-speed processing is desired in engine control or the like.
Since synchronizing signal generator means and a transmission control unit are required for preventing data collision, the construction of the data transmission system becomes complicated.
Further, optical fiber is often used as the transmission path between microcomputers (refer to the U.S. Pat. No. 5,025,142, and optical beam splitters such as half mirrors or bidirectional distributors are required in order to connect signal processing units such as microcomputers to the optical fiber transmission path and distribute a signal to them. If these distributors are inserted into the optical fiber transmission path, the optical signal remarkably attenuates and repeating amplifiers are required, which leads to a complicated structure and high cost.
Here, apart from the computer performing data sending, data transmission delay can be avoided if such register as described in the above official gazette is not provided in a computer allowing the data to pass through, but, when the data passes, for instance, through a gate or the like other than the register, the data pulse width, its shape, or the like may vary during the time of passing. Such pulse change becomes more remarkable as the number of the computers constituting the data transmission system increases, and in an extreme case pulses may disappear or they may not be discriminated from the adjacent pulses. As a result, data transmission may not be accurately performed.